I already own a chunk of aerogel (aka "frozen smoke"), which I pull from its little plastic container every couple of years to marvel at. I keep it in a bedside table drawer that's become my de facto cabinet of curiosities. My next addition to the cabinet is a chunk of gallium, because it's a metal that liquifies at 85.59°F so it will melt in your hand. It's not extremely toxic, like mercury, which I played with a lot when I was a kid, anyway.
I've been interested in gallium after reading that some people make practical joke spoons with gallium that melt when you place them in a hot beverage:
Amazon has a 20g piece of gallium for $(removed) (including shipping) and one Amazon reviewer said it the size of 3 quarters.